RU80069U1 - HETEROEPITAXIAL STRUCTURE FOR FIELD TRANSISTORS - Google Patents

Abstract

The utility model relates to semiconductor devices, namely to epitaxial structures for field effect transistors. The heteroepitaxial structure for field effect transistors contains a sequentially arranged substrate, a buffer layer, the first doped with a donor or acceptor impurity and undoped layers of a wide-gap semiconductor, an undoped layer of a narrow-gap semiconductor, undoped and second doped layers of a wide-gap semiconductor. On both sides of the narrow-gap semiconductor in the undoped layer, the third and fourth layers are made. The layers are alloyed with a different type of impurity than the first and second alloyed layers. Preferably, the second layer is alloyed uniformly in volume, and the rest is modulation. It is advisable that a modulated-doped layer doped with the same impurity as the second is made between the second volume-doped and the nearest unalloyed layers. Modulated doping may be in the form of delta doping. It is preferable that the doping level and layer thicknesses are selected so that the energy jump of the bottom of the conduction band at the heterointerface of the wide-gap semiconductor and narrow-gap semiconductor is from 0.8E _g to 1.2E _g , where E _g is the band gap of the narrow-gap semiconductor, but does not exceed band gap of a wide-gap semiconductor. The utility model allows to increase the mobility of the main charge carriers in the channel and increase the reliability and efficiency of the powerful field-effect transistors in which it is used. 4 s.p. f-ly, 5 ill.

Description

Known heteroepitaxial structure for field effect transistors containing a substrate, a buffer layer, doped and undoped layers of semiconductors (see patent JP 8055979, CL. H01L 29/812, publ. 02.27.1996). A disadvantage of the known device is the low mobility of the main charge carriers, and, as a consequence, the relatively low efficiency of operation, as well as the insufficient reliability of the transistor made on the basis of such a structure.

Known heteroepitaxial structure (HES) for field effect transistors, containing a sequentially arranged substrate, a buffer layer, the first doped with a donor or acceptor impurity and undoped layers of a wide-gap semiconductor, a layer of narrow-gap semiconductor, undoped and a second doped with the same impurity, as the first, layers of a wide-gap semiconductor (see SCWang, JSLiu, K.S. "IEEE Electron Device Letters, vol. 21, No. 7, July 2000. M. Kuzuhara, S. Tanaka" GaAs-based high-frequency and high-sp eed devices "JSAP International No. 7 January 2003).

After the manufacture of the transistor based on the well-known hydroelectric power station, a narrow-gap semiconductor layer, the band gap of which is less than the wide band gap, performs the function of the transistor channel. In the known hydroelectric power station, alloying of the structure layers is carried out

the main alloying impurity is of only one type. To obtain the electronic conductivity of the channel layer made of a narrow-gap semiconductor, the main dopant of the donor type is used.

The essence of the known method of modulated doping with the main alloying impurity is that when growing hydroelectric power plants, doping is carried out only with layers of a wide-gap semiconductor and with only one type of impurity. Also, with modulated doping of hydroelectric power plants for field-effect transistors, part of the wide-gap semiconductor adjacent to the heterointerface is not doped with the main dopant (distribution N _D (x)). The undoped layer of a wide-gap semiconductor adjacent to the heterointerfaces is called the spacer.

As a rule, the doping of a part of a wide-gap semiconductor layer with a main dopant of the same type (donor for field effect transistors with an electronic type of channel conductivity) is carried out uniformly over the wafer area. Doping over the thickness of a part of a wide-gap semiconductor layer can be both homogeneous and inhomogeneous. In particular, solutions are known for the use of delta-doping (δ doping) with the main dopant, a combination of uniform thickness doping and δ-doping with the main dopant.

In modulated doping, part of the mobile charge carriers (electrons) passes from the doped region of the wide-gap semiconductor to the narrow-gap semiconductor. This transition leads to the enrichment of the channel of the field-effect transistor with electrons.

Modulated doping leads to an increase in the mobility of charge carriers in the channel due to the elimination of their scattering by the main dopant in a narrow-gap semiconductor, since the narrow-gap semiconductor layer is not doped. Also mobility

increases due to a significant decrease in the probability of scattering by spatial fluctuations of the potential of the main dopant in the doped layer of a wide-gap semiconductor (potential fluctuations quickly decrease with distance from the doped regions towards the narrow-gap semiconductor layer).

A disadvantage of the known device is the following.

Many heterojunctions based on A ₃ B ₅ compounds, for example, Al _x Ga _1-x As-GaAs, are characterized by a relatively small value of φ _b, the jump in the energy of the bottom of the conduction band at the heterointerface between wide-gap and narrow-gap semiconductors. The characteristic value of φ _b for such heterojunctions is 0.2–0.4 eV, which is insufficient for localization of the electron gas in the layer of the transistor channel under the condition of strong thermal field heating of electrons, which takes place in the channel of the field-effect transistor made on the basis of such hydroelectric power stations.

The energy acquired by the electrons in the channel layer of the field-effect transistor as a result of thermo-field heating can significantly exceed the value of φ _b and even turn out to be equal to the energy of the forbidden band in the narrow-gap semiconductor E _g . For example, for the Al _x Ga _1-x As-GaAs heterojunction, Е _g2 in GaAs is 1.42 eV, and φ _b at x = 0.3 is approximately 0.33 eV.

Using different variants of modulated doping with one type of the main dopant does not make it possible to increase φ _b so that the localization of the main charge carriers in the channel layer of the transistor is ensured with strong thermo-field heating of the electrons. Loss of localization of the main charge carriers (for example, which is typical for most practical applications, electrons) in the channel layer of the transistor leads to the possibility of electrons scattering on the potential of the main dopant in the doped wide-gap layer and on defects in the buffer layer of the HES. Thus, when

sufficiently strong thermo-field heating of the electron gas, which is characteristic for the operation of a field-effect transistor, the advantages underlying the principle of modulated doping are lost.

The combination of the scattering effects of the main charge carriers at the potentials of the main dopants and defects leads to a significant decrease in the mobility of the main charge carriers, which is the main disadvantage of the known solution.

The objective of the utility model is to eliminate this drawback. The technical result is achieved in increasing the mobility of the main charges in heterostructures, as well as in increasing the efficiency and reliability of powerful field-effect transistors based on it. The problem is solved, and the technical result is achieved by the fact that the heteroepitaxial structure for field-effect transistors contains a sequentially arranged substrate, a buffer layer, the first doped with a donor or acceptor impurity and undoped layers of a wide-gap semiconductor, an undoped layer of a narrow-gap semiconductor, undoped, and a second doped with the same impurity, as the first, layers of a wide-gap semiconductor, while on both sides of the narrow-gap semiconductor forming a channel , in the unalloyed layer, the third and fourth layers are doped with a different type of impurity than the first and second alloyed layers. Preferably, the second layer is alloyed uniformly in volume, and the rest is modulation. It is advisable to grow between the second volume-doped and the nearest unalloyed layers a modulated-doped layer doped with the same impurity as the second. Modulated doping may be in the form of delta doping. Preferably, the doping level and layer thicknesses are chosen so that the energy jump of the bottom of the conduction band at the wide-gap heteroboundary

semiconductor and narrow-gap semiconductor was equal to from 0.8 E _g to 1.2 E _g , where E _g is the band gap of the narrow-band semiconductor, but did not exceed the band gap of the wide-band semiconductor.

Figure 1 shows the main structural elements of the known double hydroelectric power station and a possible doping profile of the main alloying donor impurity N _D (x).

Figure 2 shows the zone diagram of a known double hydroelectric power station near one heterojunction.

Figure 3 shows the main structural elements of the proposed double hydroelectric power station with additional layers.

Figure 4 shows the band diagram of the semiconductors that make up the proposed structure, before the formation of heterojunctions.

Figure 5 is the same after the formation of heterojunctions.

In the drawings and in the text, the following notation:

E _c , E _ν , F - respectively, the positions of the bottom of the conduction band, the ceiling of the valence band and the Fermi level in wide-gap and narrow-gap semiconductors after the formation of a heterojunction;

φ _b , is the value of the jump between the positions of the bottom of the conduction band at the heterointerface of wide-gap and narrow-gap semiconductors;

Е _g2 is the band gap in a narrow-gap semiconductor;

E _g1 is the band gap in a wide-gap semiconductor;

α is the width of the near-surface volume-doped layer.

Common structural elements are indicated by the following positions:

1 - wide-gap semiconductor;

2 - narrow-gap semiconductor;

3 - heteroboundaries;

4 - surface of the structure.

According to the utility model, the heteroepitaxial structure for field-effect transistors contains the following successive layers: substrate, buffer layer, donor δ (delta) layer (5) with doping level δN _D5 , undoped layer (6), layer of acceptor δ layer (7) with doping level δN _A7 , undoped spacer, undoped channel layer, undoped spacer, acceptor δ layer (8) with doping level δN _A8 , undoped layer (9), donor δ layer (10) with doping δN _D10 and volume doped donor and a layer (11) with a doping level N _D11.

HPPs are made as follows.

1. In the process of growing hydroelectric power plants, layers of wide-gap semiconductors are formed (1) modulated by the main dopants of the donor and acceptor types.

2. Carry out excessive doping of the aggregate of layers made of wide-gap semiconductors (1) with the type of main dopant that is necessary to obtain the desired type of channel layer conductivity.

3. The profile of modulated doping of the layers of wide-gap semiconductors (1) with donor and acceptor-type impurities is chosen so that the potential relief formed by the charges of these impurities provides the value of the energy jump of the bottom of the conduction band at the heterointerface (3) between wide-gap (1) and narrow-gap (2) semiconductors close to or equal in magnitude to the band gap in a narrow-gap semiconductor (2) - φ _b = (0.8 ÷ 1.2) · Е _g2 .

4. To obtain the electronic type of conductivity of the channel layer and the magnitude of the energy jump of the bottom of the conduction band at each heterointerface (3) between wide-gap (1) and narrow-gap (2) semiconductors, which is close or equal in magnitude to the band gap in narrow-gap (2)

semiconductor, in the layer of wide-gap (1) semiconductor formed on the buffer layer, the following additional layers are formed:

- a layer of wide-gap semiconductor doped with donors (5),

- a layer of undoped wide-gap semiconductor (6),

- a layer of wide-gap semiconductor doped with acceptors (7),

- it is useful that the layer (5) be delta doped with donors,

- it is useful that the interlayer (7) be delta doped with acceptors

5. To obtain the electronic type of conductivity of the channel layer and the magnitude of the jump (tear) of the energy of the bottom of the conduction band at each heterointerface between wide-gap and narrow-gap semiconductors, which is close to or equal in magnitude to the band gap in a narrow-gap semiconductor, in a wide-gap semiconductor layer formed on a layer made of unalloyed narrow-gap semiconductor, the following additional layers are formed:

- a layer of wide-gap semiconductor doped with acceptors (8),

- a layer of undoped wide-gap semiconductor (9),

- a layer of wide-gap semiconductor doped with donors (10),

- a layer of wide-gap semiconductor doped with donors (11),

- it is useful that the interlayer (8) be delta-doped with acceptors,

- it is useful that the layer (10) be delta doped with donors,

- it is useful that layer (11) be volume-doped with donors.

6. With the electronic type of conductivity of the channel layer, it is useful that the number of donors in the layer (11), referred to the area of the structure, not exceed the density of negatively charged surface states on the surface (4) of the layer (11).

7. With the electronic type of conductivity of the channel layer, it is useful that the level of doping with acceptors of the interlayer (7) and the thickness of the layer (6) be

chosen so that φ _b1 - the value of the energy jump of the bottom of the conduction band at the heterointerface 3a between wide-gap and narrow-gap semiconductors was close in magnitude to the band gap in the narrow-band semiconductor Е _g2 , but smaller than the band-gap in the wide-band semiconductor Е _g1 .

8. In the case of the electronic type of conductivity of the channel layer, it is useful that the doping level of the interlayer (8) and the layer thickness (9) be chosen so that φ _b2 , the energy jump of the bottom of the conduction band at the heteroboundary 3b between narrow-gap and wide-gap semiconductors, is close in magnitude to the band gap in a narrow-band semiconductor E _g2 , but less than the band gap in a wide-band semiconductor E _g1 .

9. It is useful that the thickness of the narrow-gap semiconductor layer be small enough for the size-quantum effect to occur in the potential well formed in the narrow-gap semiconductor by jumps in the potentials of the heterointerfaces 3a and 3b.

The principle of achieving the benefits of the proposed hydroelectric power station.

The achievement of the advantages of the proposed double hydroelectric power station is illustrated using Fig.3-5. The drawings do not show the layers necessary for the formation of ohmic contacts of the field effect transistor, since these layers do not affect the formation of the potential relief of heterojunctions.

When constructing the band diagram (see Fig. 4), it was taken into account that the magnitude of the jumps in the energies of the bottom of the conduction band at the heterointerfaces Δχ ₁ and Δχ ₂ between wide-gap and narrow-gap semiconductors under the condition of local electroneutrality of the contacting semiconductors is determined only by the difference in the electronegativities of the contacting semiconductors. Formation of a built-in potential relief in wide-gap semiconductors by conducting

modulated doping with a donor and acceptor impurity allows one to significantly change the energy jump of the bottom of the conduction band at each heterointerface between wide-gap and narrow-gap semiconductors. As a result of the addition of the built-in potential relief formed by the charges of the main alloying impurities with the potential relief of the band diagram of locally electrically neutral wide-gap semiconductors, a potential relief is formed in them that makes it possible to obtain large heterogeneous boundaries instead of Δχ ₁ and Δχ ₂ (in comparison with Δχ ₁ and Δχ ₂ ) the magnitude of the jumps in the energies of the bottom of the conduction bands, equal to φ _b1 and φ _b2 .

The charges of layers (5), (7) and (8), (10) form electric fields in undoped layers (6) and (9). By varying the thickness of the undoped layers (6) and (9), it is possible to form such potential drops on layers (6) and (9) that allow us to obtain sufficiently high potential barriers φ _b1 and φ _b2 , (see Figs. 4, 5) for electrons located in the channel layer, which is formed by a narrow-gap semiconductor. The thickness of the undoped layers (6) and (9) and the thickness of the spacers are chosen large enough to suppress the effect of electron tunneling from the narrow-gap semiconductor layer to the wide-gap semiconductor layers.

At a negligible concentration of the main charge carriers in the wide-gap semiconductor layers, the potential drop across the layers (6) and (9) is proportional to the thicknesses of these layers. Thus, the technologically set values of the charges of the layers (5), (7) and (8), (10) together with the values of the thickness of the layers (6) and (9) constitute a set of independent parameters, changes of which allow changing the height of potential barriers in the right direction φ _b1 and φ _b2 .

In the proposed solution, the thicknesses of layers (6) and (9) together with the density of acceptors in δ-doped interlayers (7) and (8) for given values of the donor density in δ-doped layers (5) and (10)

are selected so that the heights of potential barriers φ _b1 and φ _b2 satisfy the condition [1], which allows to provide the advantage of the proposed double hydroelectric power station:

The fulfillment of the inequality included in condition [1] is necessary to prevent the presence of an unacceptably high concentration of minority charge carriers (holes for a hydroelectric station in which the electronic conductivity of the channel layer is provided) in a hydroelectric power station. In this case, the presence of either an acceptor or donor type in the structure of the background dopant is admissible, the volume concentration of which is approximately equal to 1 · 10 ¹⁴ -1 · 10 ¹⁵ cm ^-3 .

Fulfillment of the approximate equality Е _b2 иφ _b1 and φ _b2 , which is part of condition [1], is necessary to maintain the localization of the main charge carriers in the layer of the channel of the structure under the condition of their strong thermo-field heating. When condition [1] is satisfied, almost all the main charge carriers remain localized in the channel layer if the energy A £ acquired by them from the electric field and as a result of thermal heating satisfies the condition [2]:

In condition [2]: k is the Boltzmann constant, T is the temperature.

The values δN _D5 , and δN _D10 , i.e. of donor densities in δ-doped layers (5) and (10) is carried out based on the value of N _e - necessary for the practical application of electron density in the layer of the HES channel. The values of δN _D5 , and δN _D10 , are determined on the basis of the electric neutrality condition of the hydroelectric power station (3), taking into account the inequality [4]:

In expressions [3] and [4], N _e is the electron density necessary for practical application in the layer of the HES channel (2), N _S is the density of charged surface states of the structure on the surface (4) of layer (11).

At present, the technology of δ doping allows one to obtain a sufficiently large value for the fulfillment of practical applications of the maximum density of donors and acceptors in δ doped layers. In particular, a maximum donor density of approximately 2 · 10 ¹³ cm ^-2 and a maximum acceptor density of approximately 8 · 10 ¹² cm ^{-2 are} achievable, while maintaining acceptable crystal lattice perfection, which makes it possible to obtain the required sufficiently large N _e .

The requirement of fulfilling the inequality [4] is given so that in the layer of a wide-gap semiconductor doped with donors there is no electrically neutral region with electron conductivity, which is an additional (spurious) channel.

Since the proposed double HES implies inequality [4], the filling of the channel layer with electrons occurs as a result of the transition of electrons localized near layers (5) and (10) into the channel layer of the structure formed from a narrow-gap semiconductor.

When optimizing the thickness of a wide-gap semiconductor layer thickened by donors according to the inequality [4] as applied to field-effect transistors with a Schottky barrier, it is taken into account that the density of charged surface states of the structure, N _S, as a rule, does not coincide with the density of charged states of the Schottky barrier. In this case, the inequality [4] is replaced by the value of the density of charged states of the Schottky barrier instead of N _S.

With excessively strong δ doping with donors of layers (5) and (10), an undesirable situation may arise when not all electrons localized near layers (5) and (10) pass into the layer of the channel of the structure. AT

In this case, additional (parasitic) channels are also formed near layers (5) and (10). The requirement of the absence of additional (spurious) channels limits the values of δN _D5 and δN _D10 from above.

When the built-in potential relief is formed in the structure, some of the electrons from all the electrons of the donor impurity go over to the energy levels of the acceptors of the interlayers (7) and (8), forming their negative charge state. Figure 5 shows the case when the layers of wide-gap semiconductors of mobile charge carriers are negligible, there are no spurious channels. In this case, the dependence of the potential relief formed in the wide-gap layers of the hydroelectric power station is ensured only on the doping profile of wide-gap semiconductors with the main alloying impurities of the donor and acceptor types.

The use of such a heterostructure in high-power field-effect transistors makes it possible to increase their reliability and efficiency by increasing the mobility of the main charge carriers in the channel.

Claims (5)

1. A heteroepitaxial structure for field-effect transistors containing a sequentially arranged substrate, a buffer layer, the first doped with a donor or acceptor impurity and undoped layers of a wide-gap semiconductor, an undoped layer of a narrow-gap semiconductor, undoped, and a second doped with the same impurity as the first, layers of a wide-gap semiconductor, characterized in that on both sides of the narrow-gap semiconductor forming the channel of the transistor, in the undoped layer, the third and fourth interlayers are made shoes doped with a different type of impurity than the first and second doped layers. 2. The heteroepitaxial structure according to claim 1, characterized in that the second layer is doped uniformly in volume, and the rest is modulated. 3. The heteroepitaxial structure according to claim 2, characterized in that between the second body-doped and the next undoped layers, a modulated-doped layer doped with the same impurity as the second is grown. 4. The heteroepitaxial structure according to claim 2 or 3, characterized in that the modulated doping is made in the form of delta doping. 5. The heteroepitaxial structure according to claim 1, characterized in that the doping level and layer thicknesses are selected so that the energy jump of the bottom of the conduction band at the heterointerface of the wide-gap semiconductor and narrow-gap semiconductor is from 0.8 to 1.2E _g , where E _g - the band gap of a narrow-band semiconductor, but does not exceed the band gap of a wide-band semiconductor.